The objective of this propoal is to study how developing nerve cell bodies and their processes establish the cytoarchitecture and the axonal pathways in the central nervous system. The investigations will be done primarily in the spinal cord of rat. In this complex process of development, we have identified factors that can be quite precisely determined. It will be determined when and where the various cells are "born", migrate, and settle in their final positions. It will be determined when and where the processes (axons and dendrites) of various populations of neuroblasts grow, assemble and terminate. Knowing the time and space parameters of the above components, we will investigate cell interactions. Based on the above, the proposal involves three levels of organization: 1) cell patterning, 2) axonal patterning, and 3) cell interactions. These observations will establish the extent to which temporal sequencing of events imparts organization to this developing system. They will also provide basic information for future investigations into the mechanisms involved. This work is also basic to investigations of health related problems such as birth defects (spinal bifida), teratogenesis, and spinal cord injury. The following techniques will be used: (H3)-thymidine autoradiography, (H3)-leucine autoradiography of axonal flow, special cytochemical stains for electron microscopy, Golgi stains, and tissue culture (dissociation-reaggregation).